Lithospheric Control on Spatial Patterns of Active Faulting in the Southeastern Sierra Nevada, California

Patterns of active faulting in the southeastern Sierra Nevada of California reflect both far-field plate motion as well as localized forces that drive seismogenic deformation. Oblique divergence between the Sierra and the western Cordillera results in an overall pattern of dextral shear and northwest-directed extension in the eastern California shear zone (ECSZ) and southern Walker Lane belt. Within the nominally rigid southern Sierra Nevada block, newly recognized active normal faulting, as well as seismicity, indicate primarily extensional deformation beneath the high topography of the southern range. Investigations of the northern Kern Canyon fault, the Little Lake fault, and the Sierra Nevada range-front faults in Rose Valley combine data from both aerial and ground-based laser scanning, paleoseismic trenching, geologic and geomorphic mapping, and surface exposure dating to define spatial and temporal patterns of fault slip. Taken together, these studies indicate that deformation kinematics along the southeastern Sierran escarpment undergo a pronounced shift at an approximate latitude of 36.5° N. To the north in Owens valley, range-front faults accommodate active extension and normal faulting, while the adjacent Owens Valley fault displays primarily dextral strike-slip motion. South of Lone Pine, however, a component of active normal faulting steps westward into the southern Sierra Nevada block to the Kern Canyon fault, while range-front faults in Rose Valley accommodate a significant component of oblique dextral extension. Focal mechanism inversion of earthquakes in the southern Sierra Nevada reveals a zone of horizontal extension and vertical crustal thinning coincident with this westward shift of normal faulting into the range. The zone of extension is directly east of the "Isabella Anomaly," a zone of anomalous high P-wave mantle velocities thought to reflect convectively downwelling or foundering lower Sierran lithosphere below the Central Valley. As such, we hypothesize that localized stresses associated with foundering lithosphere, and possibly upwelling asthenosphere, interact with far-field tectonic forces to control patterns of upper crustal faulting both within and along the eastern boundary of the southernmost Sierra Nevada.